Many micro electromechanical system (MEMS) devices require reliable vacuum volumes in order to obtain the best possible functionality and high reliability. Examples of such MEMS devices are bolometers for infrared (IR) sensing, resonant structures, and absolute pressure sensors. In general, low pressure is necessary to obtain high Q-factors in resonant structures, and is also needed in IR bolometers to obtain low noise.
The sealing of cavities, that contain the components of such systems, under vacuum typically results in pressures in the cavities in the range 1-10 mBar. However, the residual gas pressure in a hermetically sealed cavity depends on several factors, including the temperature and the materials used in the sealing process. In order to obtain reliable pressures below 1 mBar, gettering methods are usually required. It is extremely difficult, however, to obtain and maintain a suitable and reliable vacuum in MEMS devices.
For most micro system devices requiring a vacuum the disadvantages of implementing a separate vacuum sensor have been considered so critical that no such sensor has been part of the final product, even in devices where vacuum quality is paramount. In most cases, as the measurement of the actual pressure inside the cavity is cumbersome, expensive and indirect, the pressure is simply not known. The provision of a low cost but accurate vacuum sensor for MEMS devices is therefore required in the field of MEMS technology.
The implementation of a vacuum sensor into the cavity has been attempted; however, this can degenerate the vacuum level due to out-gassing from applied materials. Known devices generally utilize some sort of diaphragm (for isolation) with integrated resistive heaters and thermistor materials. In most cases these structures are both fairly large and not very well thermally isolated. They also require a particular design, often in conflict with, strongly influencing, or adding to the cost of the primary device that requires the vacuum cavity. Additionally, the thermal coefficient of resistance (TCR) of the thermistor material is not optimal in many cases, typically being in the range of −0.2 to −0.3%/K. Therefore, there exists a need to provide light-weight, compact and accurate vacuum sensors at low cost, the implementation of which causes minimal disruption to the structure, manufacture and working of the MEMS device.